1. Field of the Invention
The present invention relates to a fluid pressure equipment such as a fluid switching valve, a fluid-pressure cylinder and the like and particularly to a fluid pressure equipment provided with a low-sliding packing with small sliding resistance.
2. Description of the Related Art
In fluid pressure equipments such as an electromagnetic valve, a fluid-pressure cylinder and the like, a packing is mounted on an outer periphery of a sliding member such as a spool, a piston and the like such that the sliding member slides in a flow-passage hole through the packing.
The packing is offered in various types such as those with a circular sectional shape like an O-ring, those with a partially constricted gourd shape, Y-shape or U-shape and the like, and they are used according to the type, use conditions and the like of the fluid pressure equipment. Among them, since a packing with a gourd-shaped section has large stretching properties in a radial direction, its sliding resistance is smaller than that of the O-ring, and its sealing performance is also excellent, it is widely used in the spool, piston and the like. For example, Japanese Registered Utility Model No. 2537236 discloses a spool valve, which is one type of the fluid pressure equipment, in which a partially constricted gourd-shaped packing is mounted on the outer periphery of the spool, which is a sliding member.
However, since the prior-art gourd-shaped packing has large stretching properties in the radial direction, if a fluid pressure acts on one side face and the packing is pressed onto a groove side wall of a packing housing groove, the packing is compressed and stretched in the radial direction, and seal faces on the inner and outer peripheries are strongly pressed into contact with a target portion, which increases a squeeze amount. The increase in the squeeze amount increases sliding resistance of the packing, which leads not only to deterioration of response of the spool, that is, the sliding member, but also to fixation of the packing when the sliding member stands still, delay in start when the sliding member is switched and the like and they further lead to deterioration in reliability of the fluid pressure equipment.